Aortocaval compression by the gravid uterus during the third trimester contributes to decreased venous return to the heart. Neuraxial anesthesia reinforces this hypotension by causing a vasodilatation and venous pooling of blood in the lower limbs. The current practice is to tilt the parturient 15 degrees on the operating table after neuraxial anesthesia in order to decrease this hypotension. Recent meta-analysis suggests there is no conclusive evidence to support the tilt position. The goal of our study is to compare ultrasound vena cava variation measurements in the supine versus the tilt position in third trimester parturients undergoing elective cesarean delivery.

Cesarean delivery (CD) is one of the most currently practiced surgeries in the world. In 2013, surgical deliveries represented 32.4% of births in the United States and 26.9% in Canada. Maternal hypotension is a frequent adverse event during CD under spinal anesthesia and can be detrimental to the fetus and mother. Many means have been studied to decrease the incidence of hypotension in this context such as crystalloid/colloid pre-loading and co-loading, vasopressors, and positioning.

Aortocaval compression of the inferior vena cava (IVC) by the gravid uterus is a major contributor to this hypotension and many positions on the operating table have been researched to decrease its influence.Current recommendations for term women undergoing cesarean delivery include left lateral 15 degree tilt in order to reduce aortocaval compression which may cause maternal hypotension and fetal compromise. However, a recent meta-analysis did not show conclusive evidence to favour the tilted versus the supine position. This is probably due to a compensatory mechanism involving venoconstriction of the lower limbs, raising venous pressure and causing flow through collateral channels. Supine hypotensive syndrome with clinically significant effects occurs in 8-10% of women at term, possibly due to less compensatory mechanisms.

Respiratory variations of the IVC diameter measured by an ultrasound may determine the best position on the operating table for term parturients during cesarean delivery. Large variations of IVC diameter during expiration and inspiration have been shown to be related to hypotension in non-pregnant patients.

The primary objective of our project is to compare the collapsibility index of the inferior vena cava during elective cesarean delivery after the administration of spinal anesthesia with a phenylephrine infusion. Each participant will be their own control.

20 patients scheduled for elective cesarean delivery will be included. Each participant will be her own control for the ultrasound measurements of the inferior vena cava, with and without a tilt before and after spinal anesthesia with a phenylephrine infusion. A transcutaneous oximetry monitor (NIRS) will be used in order to measure the upper versus lower body saturation difference. The newborn Apgar score and umbilical cord pH will be noted.

Inclusion Criteria:

• ASA 1-2
• Elective cesarean delivery
• Spinal anesthesia
• At least 37 weeks of gestational age
• French speaking patients (able to read and sign the consent form)

Exclusion Criteria:

• Inability to obtain adequate ultrasound mesures before or after spinal anesthesia
• Cardiopathy
• Unexpected difficult spinal anesthesia requiring general anesthesia
• Unexpected complications requiring strong hemodynamic support (transfusions, volume challenges, multiple vasopressors, inotropic drugs...) or requiring anti-hypertensive medication (including magnesium)
• Any contraindication or patient's refusal for spinal anesthesia (e.g. coagulopathy)
• Morbid obesity (IMC over 40 at the time of delivery)
• Active labour
• Emergency cesarean delivery
• Fetal abnormality or prematurity (under 37 weeks of gestational age)
• Multiple gestation
• Inability to cooperate due to langage or physical/mental incapacity

Third trimester parturients with a normal pregnancy and a planned cesarean delivery under spinal anesthesia